Formic acid is a versatile commercially used reagent of the chemical industry. It is essential for many chemical applications and could also be used in the future—assuming an efficient preparation—for storage of hydrogen and/or carbon monoxide within the energy and chemical industries. Formic acid is industrially obtained by means of the catalytic conversion of carbon monoxide with sodium hydroxide or methanol, and subsequent hydrolysis. This process is insofar disadvantageous, however, since the purification of aqueous formic acid is problematic because formic acid forms an azeotrope and has a tendency to dissociate again at high temperatures.
The preparation of formic acid by hydrogenation of CO2 was proposed as an alternative. This reaction is indeed problematic because the ΔG for this reaction is positive, that is, a preparation is not thermodynamically possible purely from the educts. The hydrogenation of CO2 is possible if a salt or adduct formation of the produced formic acid is carried out by adding stabilizers, generally bases. Several methods have been proposed to accomplish this purpose, such as, for example, in Zhang et al, Angewante Chemie, 47, 2008, p. 1127-1129, in which an excess of ionic fluid is utilized.
All of the methods according to the prior art currently require the use of (over)stoichiometric amounts of stabilizer in the form of a base. Only formic acid salts or base adducts are formed thereby. These must be cleaved in a separate method step in order to release pure formic acid. They present thus substantially the same problems during the separation of formic acid from the used base as the above methods, in which formic acid is prepared from carbon monoxide. The used catalyst must additionally be separated in most methods according to the prior art due to toxicological and economical reasons as well as for the stability of the product. These separation and processing steps make the previous methods unattractive from the energetic and economical point of view.
Thus, there is a need to find a method for producing formic acid by means of CO2 hydrogenation, which is able to overcome for the most part the disadvantages described above and by means of which an efficient, continuous reaction control is possible.